The present invention relates to angular rate sensors and, more particularly, to apparatus and techniques for providing high resolution output signals representative of increments of angular velocity and/or increments of angular rotation.
Ring laser gyroscopes or inertial integrating rate sensors are generally known, one of which is described in U.S. Pat. No. 3,373,650 to J. E. Killpatrick and assigned to the assignee of the present invention, and hereby incorporated herein by reference. The ring laser gyroscope described in this patent makes use of two quasi-monochromatic beams of light that are generated in two opposite directions around a closed loop path about an axis of rotation in an optical resonator. Rotation about the axis causes the effective path lengths for the two beams to change, thus producing a frequency difference between the two beams since the frequency of oscillation of a laser is dependent upon the length of the lasing path.
The frequency difference between the two beams causes a phase shift between these beams which changes at a rate proportional to the rate of change of angular rotation; thus, the accumulated phase shift over time between the two beams is proportional to the integral of the rate of angular rotation. In other words, the integrated rate sensor output is representative of the integral of the input angular rate.
The phase difference between the two quasi-monochromatic beams is typically determined using a beam combiner. The beam combiner produces an interference or fringe pattern which impinges upon a pair of photodetectors. The fringe pattern represents the beat frequency of the heterodyned optical frequencies of the quasi-monochromatic beams. This fringe pattern consists of a pattern of alternate light and dark bands of light which move to the left or right depending on the direction of rotation of the gyro or angular rate sensor. If these two detectors are positioned one-quarter wavelength apart, then two sine waves 90.degree. out of phase are obtained and their relative phase lead or lag is an indication of the direction in which the laser gyroscope is rotating, such as shown in U.S. Pat. No. 3,627,425 issued to B. Doyle and assigned to the assignee of the present invention and incorporated herein by reference.
However, at low rotation rates when the difference in frequency between the two beams is small, the two beams tend to resonate together or "lock-in" so that the two beams oscillate at only one frequency. Thus, measuring low rotation rates with the gyroscope becomes impossible because expected frequency differences between the beams, proportional to the rotation rates, do not exist at these low rates.
To avoid or reduce the effects of lock-in, laser angular rate sensors may be biased as described in U.S. Pat. No. 3,373,650. This biasing technique, usually referred to as "dithering", much reduces the lock-in problem by operating the laser angular rate sensor in a manner such that it is not required to measure low input rates directly. That is, the resonator is electrically or mechanically oscillated with respect to a base so that the resonator seems to be rotating at a rate higher than the lock-in rate for a majority of the time. The times when the resonator is below the lock-in rate, at the extremities of the oscillation, are only very small fractions of the dither oscillation period and, consequently, have a relatively small effect on the operation of the sensor.
There are, however, some disadvantages to the use of biasing techniques to eliminate the effects of lock-in. As a result of either mechanical or electrical biasing, larger path length differences for the oppositely rotating beams are achieved most of the time to thereby produce a resultant frequency difference to much reduce the problem of lock-in. However, this added frequency difference is recorded by each of the photodetectors thereby adding a dither component to the integrating rate sensor output.
Each of the photodetectors provides an output signal representative of the light and dark bands produced by the interference pattern that impinges on these photodetectors. The output of each of the photodetectors is a sinusoidal signal having a frequency that represents the rate of fringe pattern movement past each photodetector. The photodetector output frequency, or rate of fringe pattern movement, is proportional to the rate of angular rotation of the sensor. Frequently, the photodetector output is provided to a zero-crossing detector that converts the photodetector output to a series of discrete pulses. These pulses can be counted in a known manner and count totals thereof are sampled by a signal processing system to determine the angular rotation rate of the sensor. The ratio of the number of discrete pulses or counts generated by a photodetector relative to the amount of rotation of the sensor is called the "scale factor." This scale factor is fixed in part by the size of the ring laser gyro or, more precisely, by the size of the closed optical path or resonant cavity, i.e. the optical resonator, which is oscillated. In addition to the size of the resonant cavity, the lasing wavelength also affects the interference pattern and therefore the scale factor of the sensor. Therefore, the resolution of an angular rate sensor output signal is limited by the resonant cavity size and the lasing wavelength in relation to the noise sources present in the gyroscope.
Inertial navigation systems usually make use of three or more inertial integrating rate sensors. These sensors usually have an orientation such that the axes of sensitivity of the inertial integrating rate sensors are substantially mutually orthogonal to one another thereby forming a basis for a corresponding coordinate system.
Frequently, an accurate determination of the attitude of such an inertial navigation system coordinate frame with respect to an earth-bound frame, or coordinate system, is needed. The attitude of the inertial navigation system at any instant in time can be determined using a digital computer in a known manner based on an initial attitude and the instantaneous angular rotations the inertial navigation system has undergone. Thus, improving the resolution of each of the inertial integrating rate sensors that are used with the navigation system improves the accuracy of the attitude angle that is computed therefrom.
Aside from inertial navigation systems, there are other applications where the integrating rate sensor that is used must provide very accurate rotational information. In these applications, sometimes only one or two integrating rate sensors are mounted to an object as a means for providing angular rotation information to a system. For example, inertial integrating rate sensors may be used to provide rotational information for an optical telescope having just one or two axes of rotation. This rotational information is provided to a control system that is capable of selectively activating servo motors to reposition the telescope, thus insuring the telescope remains pointed at a selected target.
There is a present need for a high resolution or high accuracy signal that represents either increments of angular rotation or increments of angular velocity of the sensor. Moreover, this system should be easy to implement, make use of low cost components and have high reliability. In addition, the sensor output signal produced by dithered or rotationally oscillated angular rate sensors contains a signal component that is directly related to biasing the sensor. This signal component in the sensor output signal due to the biasing is herein referred to as the dither signal component. This dither signal component represents an instantaneous error in the sensor output insofar as that signal represents the undithered angular motion of the resonator. Therefore, in many applications requiring high accuracy or high resolution, the need also exists to minimize or remove this dither signal component in the sensor output signal.